A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head, which is housed in a slider, and the surface of a magnetic-recording disk must be tightly controlled. An actuator relies in part on a suspension's force on the slider and on the aerodynamic characteristics of the slider air bearing surface (ABS) to provide the proper distance between the read/write head and the surface of the magnetic-recording disk (the “flying height”) while the magnetic-recording disk rotates.
FIG. 2 is a cross-sectional side view of a conventional write head. Write heads make use of the electricity flowing through a coil 202 in the write head 200, which produces a magnetic field. One type of coil design is referred to as a helical coil because it wraps around the write pole, e.g., main pole 208, in a helical shape. Such a write head includes a helical write coil having upper coil portions 202a, 202b, 202c that pass above the write pole and lower coil portions 202d, 202e, 202f that pass below the write pole. The upper and lower coil portions are connected with each other by connection tabs. Electrical pulses are sent to the write head, with different patterns of positive and negative currents. The current in the coil of the write head induces a magnetic field across the gap between the head and the magnetic disk, which in turn magnetizes a small area on the recording medium.
A perpendicular magnetic recording (PMR) system records data as magnetizations oriented perpendicular to the plane of the magnetic-recording disk. The magnetic disk has a magnetically soft underlayer covered by a thin magnetically hard top layer. The perpendicular write head has a write pole (main pole 208) with a very small cross section at the pole tip 208a, tapered down from the cross section along the length of the yoke 208b from which the pole tip protrudes, a lower return pole 206, and an upper return pole 218 having a much larger cross section along the length. Also shown in FIG. 2 is a stitch pole 207 for assisting in delivering the magnetic field, and a wrap-around shield 209 for assisting in focusing the magnetic field emitting from pole tip 208a. Note that with conventional write heads configured such as write head 200, the upper coil portions 202a, 202b, 202c are necessarily some distance from the stitch pole 207 and the main pole 208, separated by a relatively thick insulation material layer such as alumina or hard-baked photoresist.
A strong, highly concentrated magnetic field emits from the write pole in a direction perpendicular to the magnetic disk surface, magnetizing the magnetically hard top layer. The resulting magnetic flux then travels through the soft underlayer, returning to the return pole where it is sufficiently spread out and weak that it will not erase the signal recorded by the write pole when it passes back through the magnetically hard top layer on its way back to the return pole.
Advanced PMR writers demand high data rate write heads, especially for advanced server products. For high data rate performance, a shorter yoke length write head is faster in writing data bits, for the same total write current. Thus, the shorter the yoke length, and the higher the number of coil turns, the faster the write head. However, in conventional write heads, yoke length reduction is challenging in part because of the limited amount of space available for the coil to wrap around a shorter yoke.